USB: serial: cp210x: refactor flow-control handling

Add a helper function to be used to configure flow control.

The flow-control code was the last caller that relied on the
memset-on-failure behaviour of cp210x_read_reg_block(), which we can now
drop in favour of bailing out on errors when retrieving the flow-control
settings.

This should also simplify adding support for software flow control.

Signed-off-by: Johan Hovold <johan@kernel.org>